Change source packages download path from `src` to `firmware/src` when building for host

[jamoralp]

When executing:

ros2 run micro_ros_setup create_firmware.sh host

on a colcon workspace, required package's source files, such as eProsima's vendor packages (MicroCDR, Micro-XRCE-DDS-Client) and ros2 packages are downloaded under src. I would expect the same behaviour as when creating a workspaces for an embedded MCU, which locates everything related with the micro-ROS build under a firmware folder. Thus, deleting this folder assures the user that he/she can start from scratch, while being coherent with the rest of build cases for a micro-ROS environment.

Therefore, I propose moving the location of these files under a firmware/src folder.

Similarly, when creating the workspace for compiling the micro-ros-agent, source artifacts are downloaded under src folder. Having a dedicated uros_agent workspace folder would be desirable.